It is known to design building structures including multi-story building structures with modified foundations designed to isolate the building's superstructure from major ground motion during an earthquake. Essentially, in this prior art the superstructure is supported by its foundation so that during an earthquake relative, primarily horizontal, displacement is permitted between the foundation and the superstructure so that the high horizontal forces encountered during an earthquake will not be transferred to the superstructure in an amount sufficient to cause irreparable damage to, or destruction of, the superstructure.
Structures utilized to achieve this result include the apparatus disclosed in U.S. Pat. No. 3,638,377 dated Feb. 1st, 1972 to M. S. Caspe, U.S. Pat. No. 4,166,344 issued Sept. 4th, 1979 to A. S. Ikonomou, and U.S. Pat. No. 4,269,011 issued May 26th, 1981 to Ikonomou.
All of this known prior art is concerned in particular with building structures and teaches specific means for avoiding the translation to that structure of high seismic forces which if transmitted to the structure would be adequate to severely damage or destroy the structure, with serious consequences.
Bridge structures, as well as building structures which are located in an earthquake zone, are capable of being damaged or destroyed by seismic forces, often with serious consequences. In general bridge structures, due to their nature, are constructed with bearings to both support and guide it, located between the bridge's deck or superstructure and the bridge supporting piers or foundations to permit relative movement between the two which movement occurs primarily as a result of dimensional changes in a longitudinal direction in the bridge deck caused by temperature changes, creep, shrinkage, earth and other movements. There are many known bearings utilized to permit movement of a bridge deck relative to its supporting structure. These bearings, as is well known, can take many different forms and include sliding plate bearings, pot bearings, rotatable spherical and cylindrical bearings and high load structural bearings. They can be fixed, multidirectional or unidirectional bearings. If fixed, guide bearings must also be provided. Normally, both the supporting and guiding is accommodated by one bearing. U.S. Pat. Nos. 3,921,240 and 3,806,975 exemplify some of these known bearings.
It is also known to provide damping for the movement upon these bearings of superstructure relative to supports, however the permitted relative movement is not large and furthermore it is not always preferred to attempt to hold a superstructure in a position around a neutral point with respect to the supports.
It would be highly desirable to provide those bridges located in earthquake zones with bearing structures which function to accommodate both the normal support and/or guiding function, and when necessary, seismic forces resulting from an earthquake. In particular it would be advantageous to have an aseismic bridge bearing structure which includes means for reducing to an acceptable extent the horizontal seismic forces transmitted to a bridge superstructure during an earthquake to thereby prevent damage to the bridge superstructure, or at least reduce damage, to the degree necessary to permit the bridge to remain relatively intact during the earthquake, and permit it to be readily repaired after the earthquake.